BU21024FV-M_14 Datasheet, PDF (9/20 Pages) Rohm – Resistive Type Touch Screen Controller ICs

BU210243FGVU-LM BU21023MUV BU21024FV-M
Datasheet
Background Information
A resistive touch screen is made up of multiple layers of sandwiched resistive films and protective coatings all sitting on top
of an LCD display. Resistive touch screens work by direct contact of a stylus or a finger flexing a pair of resistive films, hence
any blunt pointing instrument or a gloved finger may be used.
Touch screen resolution
The resolution of a touch screen is typically measured in dots per inch (dpi) and is a function of the physical size of the touch
screen and the ADC used in the conversion circuitry. For example, a 3”x5” screen used with the BU21024 devices can
provide a theoretical resolution of 1024 /5 = 204 dpi. In the case of resistive touch screens, the direct contact nature of its
operation and finger thickness often impose an upper limit on the effective system resolution that may be achieved,
regardless of the resolution of the ADC itself.
Description of Function
The BU21024 devices connect to a standard 4-wire resistive touch screen on one side and to a HOST processor on the
other side. The BU21024 includes four additional sense terminals allowing it to be used with either 4-wire or 8-wire resistive
touch screens. The BU21024 include the analog and digital circuitry to process and provide dual touch coordinate data and
pinch, spread gesture information to the HOST .
The BU21024 devices include an internal CPU and provide a high degree of programmability by exposing a set of registers
that can be accessed by a HOST through SPI or I²C serial interfaces. The IFSEL pin determines whether SPI or I²C interface
is selected, The BU21024 devices include an INT pin whose polarity can be programmed via registers. When an interrupt
occurs, for example, due to touch detection, the HOST processor is required to read an interrupt status register to determine
the cause of the interrupt and take appropriate action.
The program memory of the internal CPU may be initialized via the HOST interface or via an external EEPROM. This is
selected by a register setting.
The BU21024 devices support two sensing modes; continuous and interval sensing modes, that are register- selectable. In
the continuous sensing mode, the embedded CPU reads Z, X & Y coordinates continuously when a touch is detected. The
interval sensing mode allows a delay to be inserted between each cycle.
The BU21024 devices enable optimization of touch detection threshold for a given screen. They also include several
sophisticated calibration algorithms.
Often, a description of the registers followed by flow charts requires other document to explain the steps in finer detail.
The other document also includes information on touch screen parameters that one should look for when selecting resistive
touch screens for multi-touch.
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